Wireless terminals have a transmit chain for signal transmission. Various components in the transmit chain enable a wireless terminal to control the level of power supplied to the transmitter as well as control other transmission factors. The efficiency of the wireless terminal's transmit chain impacts not only transmission quality but also battery consumption, as providing additional power to the transmit chain negatively impacts battery life. One component of the transmit chain that impacts efficiency is a power amplifier that may be used to alter the amplitude of a signal prior to transmission.
In addressing the problem of wireless terminal power amplifier efficiency, several methods have been proposed for envelope tracking or polar modulation approaches for power amplifier efficiency enhancement combined with predistortion for recovery of the waveform error vector magnitude (EVM). In either of these systems, a bias amplifier that is used to modulate the drain bias of a power amplifier with a voltage that to some degree tracks the envelope of the signal waveform. In this manner, the bias of the power amplifier is reduced when the waveform envelope is relatively small to conserve power dissipation in the power amplifier. When the waveform envelope is relatively large, the power amplifier receives full bias to support amplification of the signal. The power amplifiers in these systems are typically biased to class AB, class D, or class E.
Due to the non-linear response of the power amplifier to modulation of the drain bias, it is normally necessary to combine an envelope tracking or polar modulation transmitter with some form of predistortion in order to meet the EVM requirements at the air interface. However, this approach has limitations when implemented in a wireless terminal. For example, one limitation is that the bandwidth requirement of the amplitude modulation is typically about two to five times that of the waveform bandwidth. Therefore, for narrowband waveforms such as Global System for Mobile communications (GSM) or General Packet Radio Service (GPRS), the amplitude modulator bandwidths are practical, while for wideband waveforms such as Wideband Code Division Multiple Access (WCDMA), 802.11x, or WiMax, the bandwidth requirements on the amplitude modulator can approach thirty to fifty MHz, making the implementation of such circuits impractical.
A second limitation is the efficiency of the amplitude modulator. If a linear regulator type modulator is used, less power is dissipated in the power amplifier as the voltage is reduced to the power amplifier drain, but the additional power is dissipated in the regulator due to the higher voltage drop. The net effect to the battery power drain using a linear regulator type modulator is generally insignificant since the power saved in the power amplifier during low envelope amplitudes is merely dissipated in the regulator.
As such, a switching regulator is often used as the drain bias modulator. Such a regulator can generally maintain an efficiency of eighty percent to ninety percent over a large regulation range. However, for wireless terminal applications, the use of switching regulators can be undesirable due to such factors as the large reactive components that are often required in the circuit, the use of a greater number of components, potential electromagnetic interference (EMI) issues, prohibitive cost of the added circuit components, or spurious problems resulting from the switching noise of the circuit.
There are additional challenges to these approaches related to delays and delay variations between the drain bias modulation and the waveform envelope, dynamic range limitations, and other issues, although these challenges are not documented herein.
Accordingly, an improved system and method for controlling a power amplifier in the transmit chain of a wireless terminal are needed.